Garage conversions and annexes.

What needs permission, and what counts as a separate home.

Reviewed August 2026

The short answer

Converting an attached garage is usually not development, because the work affects only the interior or does not materially affect the outside. The rules bite when the use changes. Living accommodation in a garden building is not an incidental use, and a self-contained annexe is a separate dwelling.

In practice · Eydon

A garage that needed permission

The clearest way to answer the question is to follow one that had to be applied for.

The order of work

01Which buildingAttached, or standing on its own.

02Which useIncidental, ancillary, or a home.

03Full applicationThe only route the use left open.

04Then Building RegulationsA garage is not habitable space.

A house at Eydon, a village on the southern edge of Northamptonshire, with an existing garage the owners wanted as living space. The description of the development on the public register runs to seven words. No enlargement, no new floor area.

The job

Site: A house with an existing garage, Eydon, Northamptonshire
Role: Agent on the planning application
Consent: West Northamptonshire Council ref WNS/2021/0010/FUL, full planning permission, 11 May 2021
Outcome: Consent to convert the garage to living accommodation

Almost nothing in a job like this is development. Alterations affecting only the interior of a building, or not materially affecting its external appearance, sit outside the definition. What is development is the change in what the building is for, and that is where a garage conversion is decided.

A garage in the grounds of a house is there for a purpose incidental to the house. Living accommodation is not incidental, and the government's own technical guidance says so in as many words: an incidental purpose does not cover the use of an outbuilding for primary living accommodation such as a bedroom, bathroom or kitchen.

So there was no permitted development route and no argument that nothing was needed. The application went in on 8 March 2021 and was permitted on 11 May, sixty-four days later.

The arithmetic is in the fee. The council charges £548 to alter or extend the house, £272 for building work in the grounds that serves it, and £610 for each new dwellinghouse created by changing a building's use. Working out which of the three a project is turns out to be most of the job.

The reason the interior rule is not the whole answer is that planning looks at the planning unit rather than at the wall. A house and the land around it are ordinarily one unit, and a room inside an outbuilding that the household uses as part of the house can stay within it. What takes a building out of the unit is independence: its own kitchen, its own entrance, and the practical ability to be occupied by someone who is not part of the household. That is a question of fact, judged on how the building will actually work, and a label on a drawing does not settle it.

An attached garage is a different case again, because it is already part of the house. Turning it into a room is work to the inside and the use does not change. The front of it is the part to watch. Filling in the door opening and setting a window in its place changes the elevation, and the exemption holds only for as long as that change does not materially affect the external appearance of the house. A close match in the same brick usually stays inside it. A wholly new arrangement of openings usually does not, and the extensions guide linked at the foot of this page is the one that answers it.

The proof

Four buildings that needed an application.

Listed · two consents

Binswood Avenue, Leamington

A garage conversion and a basement conversion inside a listed villa, with a single storey rear extension. Listing reaches the inside of a building, so the internal work that would ordinarily need nothing needed a consent of its own. Two applications, lodged on the same day and decided on the same day.

Warwick District Council W/19/1602 and W/19/1603/LB · granted 25 November 2019

Detached · in front

Kenilworth Road, Coventry

A two storey detached garage standing forward of the house, with a pergola in the garden behind it. Two of the Class E tests fail here at once: nothing may stand forward of the principal elevation, and nothing may rise above a single storey. There was no route but an application.

Coventry City Council ref HH/2022/1563 · granted 30 November 2022

Garage loft · ancillary

Old Warwick Road, Lapworth

A garage loft turned into residential accommodation for use in association with the main dwelling, with a balcony to the side and a staircase at the back. Residential accommodation is not an incidental use, whatever the building it goes into, so there was no permitted development route and an application was the only way through.

Warwick District Council ref W/02/1418 · granted 5 November 2002

Outbuilding · retrospective

Stud Farm, Hill Wootton

A single storey outbuilding already standing when the application went in, put forward retrospectively and granted nine weeks later. Applying after the event is a worse position than applying before it, and it is a great deal better than leaving the question open on the file.

Warwick District Council ref W/12/1234 · granted 6 December 2012

Every planning application we make in our own name is a matter of public record on a council register. Of the entries whose description of the work survives in the record, fourteen are applications for garages, outbuildings and ancillary accommodation at existing homes, granted across seven planning authorities, the earliest of them received in 1991.

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The first step

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01 · Where you stand

Start with what the building is

Two facts about the property settle most of the answer before anything is drawn. The first is whether the garage or outbuilding is attached to the house or standing on its own. An attached garage is part of the house. A detached one is a separate building in the grounds, and different rules reach it.

The second is whether the property has any householder rights left at all. Class E reaches houses only. A building containing flats is not a dwellinghouse for this purpose, and neither is a flat inside one. Most of the prior approval routes that turn another building into a home switch Class E off as well, and the order names the ones it catches. Nothing standing in the grounds of a listed building has the right either.

Rights are also removed one property at a time. The consent that allowed the house can carry a condition taking them away, and that condition binds every later owner whether or not it came up at the sale. An article 4 direction removes them too, and the order lets a direction name one particular development, so a single property can be caught while the street around it is not. Designated land narrows what is left.

Listing catches people out here more than anything else, because a garage, a coach house or a garden building standing in the grounds of a listed house is very often listed with it whether or not anyone has ever said so out loud. Nothing in the grounds of a listed building is permitted development under Class E, and work affecting the character of the listed building needs a consent of its own on top of any planning permission.

Class E is switched off for one further category: a house built under Part 20 of the same order, which permits new dwellinghouses on and above existing buildings and in place of demolished ones. It is a small group and a growing one, and a home inside it has no garden building right at all.

02 · The routes

Four routes, and what each costs

There are four ways a garage, an annexe or a garden building reaches consent, and the building and its intended use decide between them. That decision belongs at the start, before anything is drawn, because it sets the fee and the timetable as well as the drawings.

Route 01

Nothing required

Two exemptions do most of the work here. Alterations that affect only the interior of a building, or that do not materially affect its external appearance, are not development. Nor is using a building in the grounds of a house for a purpose incidental to the house. Between them they cover a great many attached garage conversions outright.

Where it fails: the moment the outside changes materially, or the use becomes living space.

Route 02

Permitted development

Class E grants a building or enclosure in the grounds of a house, provided its purpose is incidental to the house. It must be single storey, and no part of it may stand forward of the wall forming the principal elevation of the original house. No application, and no fee.

Where it fails: a verandah, a balcony or a raised platform is excluded outright, as is anything in the grounds of a listed building.

Route 03

Full application

The route once the use is not incidental, or the building is outside the Class E limits. The fee follows what is being applied for: £272 for a building in the grounds that serves the house, £548 to alter the house itself, £610 for each dwellinghouse created by a change of use.

Where it fails: pick the wrong category and the application is not valid, so the clock never starts.

Route 04

Certificate of lawfulness

A ruling on the law, not a decision on the merits. The council is asked to certify that the proposal would be lawful, and it is bound by the certificate it issues unless the facts move before work starts. The fee is half that of the application it stands in place of.

Where it fails: it answers the planning question only. Building Regulations approval is untouched by it.

03 · The line

Incidental, ancillary, or a separate home

Planning takes little interest in what a building is called. Garden office, studio, granny flat, annexe: those are selling words. What the system asks is what the building will be used for, and there are three answers to that question.

Incidental means a use that serves the house without being part of living in it. A garage, a store, a workshop, a greenhouse, a pool. Incidental uses in the grounds of a house are the ones the Act leaves alone altogether. Ancillary is a step further in: living space that is genuinely part of the same household and the same planning unit. It usually needs permission, and it does not create a new home.

A separate dwelling is the third answer and it is a different thing entirely. The Act settles it in a line: using a building previously used as a single dwellinghouse as two or more separate dwellinghouses is a material change of use. That needs full planning permission, it counts as a new home, and the council will judge it on how the building actually works rather than on what the drawing calls it. Its own kitchen, its own entrance and the practical ability to live in it without using the main house are the things that decide it.

Whether a building is ancillary is a matter of degree rather than of design. A shared heating system, a shared drive and one garden point one way. A separate meter, a separate postal address and a front door that locks against the house point the other. No single feature is decisive, and a kitchen is the one that carries the most weight.

A council content with an annexe but not with a second house on the plot will often say so with a condition rather than with a refusal, tying the occupation of the annexe to the main dwelling. That is usually the right outcome and a workable one. It is also a permanent restriction on the property, and it repays reading before it is accepted rather than after.

04 · The second approval

A garage is not a habitable room

Building Regulations approval is a separate consent from planning permission and it applies whichever planning answer a project gets. It is the approval a garage conversion almost never escapes, because the work turns a space built to keep a car dry into one built for people.

A garage floor is usually a slab laid to fall towards the door, with no insulation beneath it and often no damp proof membrane. The walls are frequently single skin, or a cavity that was never filled. The opening where the door was is not a wall. Add ventilation, and, where the new room is a bedroom, a means of escape, and the list of things to be built rather than merely finished gets long.

The approval comes from the council's building control team or from a registered building control approver, and it is inspected as the work proceeds rather than signed off in advance. Without it the house has no compliance certificate to show a buyer.

The order in which these decisions happen matters more than people expect. A floor build up settled after the planning drawings are approved can cost a step down into the new room, or a ceiling too low to be comfortable, and neither is easy to undo once the slab is poured. The same is true of the window: the opening size that suits the elevation and the opening size that gives a bedroom its escape route are not always the same opening.

A certificate of lawful development does not touch any of this. It rules on the planning question and says nothing at all about whether the room can lawfully be built, which is a distinction worth holding on to when a builder produces one as though it were an approval.

Common questions

Garages and annexes, answered

Does converting a garage change the council tax?

It can. A self-contained annexe is usually banded separately, and where it is there is a national 50 per cent reduction on the annexe's bill.

The reduction comes from the Council Tax (Reductions for Annexes) (England) Regulations 2013, which came into force on 1 April 2014. It applies where the annexe forms part of a single property that includes at least one other dwelling, and is either occupied by a relative of the person liable for the main dwelling or used by that household as part of their own home. It reduces the bill on the annexe, not on the house, and the billing authority has to be told rather than finding out on its own. Turning a garage into a room that stays part of the house does not create a second band at all.

Is an annexe liable for the Community Infrastructure Levy?

A new annexe is chargeable development, but there is an exemption written for it, and the exemption has to be claimed.

Regulation 42A of the Community Infrastructure Levy Regulations 2010 exempts a residential annexe where the person claiming owns a material interest in the main dwelling and occupies it as their sole or main residence, and where the annexe sits wholly within the grounds of that dwelling and comprises one new dwelling. The exemption is not automatic. A claim has to reach the authority that collects the levy, and be granted, before anything starts on site. Break ground first and the relief is gone. The same regulation carries a separate exemption for a residential extension, which is the one that applies where the new space enlarges the house rather than creating a second dwelling.

Can an annexe be sold or let separately?

Only if it is a separate planning unit with a permission of its own. An annexe consented as ancillary accommodation cannot be.

An annexe permitted as ancillary accommodation is part of the same planning unit as the house, so selling or letting it on its own would be a material change of use needing permission in its own right. Councils frequently make that explicit with a condition tying the occupation of the annexe to the main dwelling. Lifting such a condition later means a further application, and the council is entitled to reach the same conclusion it reached the first time. If separate sale is ever the intention, the moment to say so is at the application, not five years afterwards when a buyer's solicitor asks the question.

Can I run a business from a converted garage?

Usually yes and without permission, provided the business use stays incidental to living in the house.

The planning practice guidance puts it plainly: permission is not needed for home working or a business use provided that use is incidental to the use as a dwellinghouse. That is Paragraph 014, Reference ID 13-014-20230726. What tips it over is the effect rather than the activity. Customers or staff arriving, deliveries, signage, noise outside ordinary hours, or a share of the property given over to the business large enough that it is no longer mainly a home, are the things a council weighs. One person working alone at a desk is nowhere near that line. A workshop with a van outside and a trade counter is.

How big can a garden room or outbuilding be?

Under Class E: single storey, 4 metres high with a dual pitched roof, 3 metres otherwise, and 2.5 metres within 2 metres of a boundary.

Eaves may not exceed 2.5 metres. The limit people miss is the other one: everything standing in the grounds other than the original house, taken together, may not cover more than half of the grounds, and that half is measured on the area of the grounds with the original house's own footprint taken out of the sum. Extensions built under the extension class count towards it as well. Original means the house as first built, or as it stood on 1 July 1948 if it is older than that, so a previous owner's sheds and additions come off the allowance before any of yours are counted.

Do the rules change in a conservation area?

They narrow. Nothing may go between a side wall of the house and the boundary, and in the national designations there is a second limit as well.

On article 2(3) land, which means a conservation area, a National Park, a National Landscape, the Broads or a World Heritage Site, Class E does not permit any part of a building, enclosure, pool or container to stand between a wall forming a side elevation of the house and the boundary. In a National Park, a National Landscape, the Broads or a World Heritage Site there is a further limit: everything sited more than 20 metres from any wall of the house may not cover more than 10 square metres in total. A conservation area is not caught by that second limit, only by the side elevation one, and the difference is worth knowing before a garden building is positioned.

The garage was converted before I bought the house. What now?

A certificate of lawfulness of existing use is how that gets settled, and the evidence is the owner's to produce.

Section 191 of the Town and Country Planning Act 1990 lets an owner apply for a certificate that an existing use or operation is lawful. It is decided on the balance of probabilities and on the applicant's evidence, which is usually a mix of dated photographs, utility bills, statutory declarations from people who were there and whatever the council's own records hold. The fee is the same as the planning application it stands in for, not half of it. A certificate is the only thing that closes the question properly, and it is far easier to obtain while the previous owner can still be asked for the paperwork.

Written by

Rickett Architects

A RIBA Chartered practice, working since 1970, with more than 2,500 projects behind it. Our senior architects take projects from first survey through planning to completion, for private clients, developers and landowners across the Midlands and well beyond it.

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